HAL Port — Implementation Details
This documentation explains how the eCos HAL specification has been mapped onto M68K hardware, and should be read in conjunction with that specification. It also describes how variant, processor and platform HALs can modify the default behaviour.
eCos support for any given target will involve either three or four HAL packages: the architectural HAL, the platform HAL, the variant HAL, and optionally a processor HAL. This package, the architectural HAL, provides code and definitions that are applicable to all M68K processors. The platform HAL provides support for one specific board, for example an M5272C3 evaluation board, or possibly for a number of almost-identical boards. The processor HAL, if present, serves mainly to provide details of on-chip peripherals including the interrupt controller. The variant HAL provides functionality that is common to a group of processors, for example all MCFxxxx processors have very similar UARTs and hence can share HAL diagnostic code. There is no fixed specification of what should go into the variant HAL versus the processor HAL. For simplicity the description below only refers to variant HALs, but the work may actually happen in a processor HAL instead.
As a design goal lower-level HALs can always override functionality
that is normally provided higher up. For example the architectural HAL
will provide the required eCos
HAL_MSBIT_INDEX macros, but these can be
provided lower down instead. Many but not all ColdFire processors have
which allow for a more efficient implementation than the default
architectural ones. In some areas such as handling context switching
the architectural HAL will usually provide the basic functionality but
it may be extended by lower HALs, for example to add support for the
multiply-accumulate units present in certain ColdFire processors.
The architectural HAL provides header files
cyg/hal/arch.inc. These automatically
include an equivalent header file from the variant HAL, for example
variant HAL header will in turn include processor and
platform-specific headers. This means that application developers and
other packages can simply include the architectural HAL headers
without needing to know about variants or platforms. It also allows
the variant and platform HALs to override architectural settings.
The port assumes that eCos and application code always runs in supervisor mode, with full access to all hardware and special registers.
For eCos purposes all M68K processors are big-endian and 32-bit, so
the default data types in
cyg/infra/cyg_type.h are used. Some
variants have external bus widths less than 32-bit, but this does not
affect the architectural HAL.
When porting to another variant it is possible to override some or all
of the type definitions. The variant HAL needs to implement the CDL
CYGINT_HAL_M68K_VARIANT_TYPES and provide
a header file
Startup and Exception Vectors
The conventional bootstrap mechanism involves a table of exception vectors at the base of memory. The first two words of this table give the initial program counter and stack pointer. In a typical embedded system the hardware is arranged such that non-volatile flash memory is found at location 0x0 so it is the start of flash that contains the exception vectors and the boot code. The table of exception vectors is used subsequently for interrupt handling and for hardware exceptions such as attempts to execute an illegal instruction. There are a number of common scenarios:
On systems with very limited memory flash may remain mapped at
location 0 and the table of exception vectors remains mapped there as
well. The M68K architecture defines the table to have 256 entries and
hence it occupies 1K of memory, but in reality many of the entries are
unused so part of the table may get used for code instead. Since the
whole exception vector table is in read-only memory parts of the eCos
interrupt and exception handling mechanisms have to be statically
initialized and macros like
HAL_VSR_SETare not available.
As a minor variation of the previous case, flash remains at location 0
but the table of exception vectors gets remapped elsewhere in the
address space, usually RAM. This allows
HAL_VSR_SETto operate normally but at the cost of increased memory usage. The exception vector table in flash only contains two entries, for the initial program counter and stack pointer. The exception vector table in RAM typically gets initialized at run-time.
On systems with more memory it is conventional to rearrange the address map during bootstrap. The flash gets relocated, typically to near the end of the address space, and RAM gets placed at location 0 instead. The exception vector table stays at location 0 but is now in RAM and gets initialized at run-time. The bootstrap exception vector table in flash again only needs two entries. A variation places the RAM elsewhere in the address space and moves the exception vector table there, leaving location 0 unused. This provides some protection against null pointer accesses in errant code.
As a further complication, larger systems typically support different startup types. The application can be linked against a ROM startup configuration and placed directly in flash, as before. Alternatively there may be a ROM monitor such as RedBoot in the flash, taking care of initial bootstrap. The user's application is linked against a RAM startup configuration, loaded into RAM via the ROM monitor, and debugged using functionality provided by the ROM monitor. Yet another possibility involves a RAM startup application but it gets loaded and run via a hardware debug technology such as BDM, and the ROM monitor is either missing or not used.
The exact hardware details, the various startup types, the steps
needed for low-level hardware initialization, and so on are not known
to the architectural HAL. Hence although the architectural HAL does
provide the basic framework for startup, much of the work is done via
macros provided by lower-level HAL packages and those macros are
likely to depend on various configuration options. Rather than try to
enumerate all the various combinations here it is better to look at
the actual code in
vectors.S and in appropriate
variant, processor or platform HALs.
responsible for any low-level initialization that needs to happen.
This includes setting up a standard C environment with the stack
pointer set to the startup stack in working RAM, making sure all
statically initialized global variables have the correct values, and
that all uninitialized global variables are zeroed. Once the C
environment has been set up the code jumps to
hal_m68k_c_startup in file
hal_m68k.c which completes the initialization and
jumps to the application entry point.
The M68K architecture reserves a 1K area of memory for 256 exception vectors. These are used for internal and external interrupts, exceptions, software traps, and special operations such as reset handling. Some of the vectors have well-defined uses. However when it comes to interrupt handling the details will depend on the processor variant and on the platform, and the appropriate package documentation should be consulted for full details. Most platforms will not use the full set of 256 vectors, instead re-using some of this memory for other purposes.
By default the exception vectors are located at location 0, but some
variants allow the vectors to be located elsewhere. This is managed by
an M68K-specific macro
default value is 0, but a variant HAL can provide an alternative value.
The standard eCos macros
HAL_VSR_SET just manipulate one of the 256
entries in the table of exception vectors. Hence it is usually
possible to replace the default handlers for exceptions and traps in
addition to interrupt handlers.
#define's for the more common exception
vectors, and additional ones can be provided by the platform or
variant. It is the responsibility of the platform or variant HAL to
initialize the table, and to provide the
HAL_VSR_SET_TO_ECOS_HANDLER macro since that
requires knowledge of the default table entries.
It should be noted that in some configurations the table of exception
vectors may reside in read-only memory so entries cannot be changed.
If so then the
HAL_VSR_SET_TO_ECOS_HANDLER macros will not be
defined. Portable code may need to consider this possibility and test
for the existence of these macros before using them.
The architectural HAL provides an entry point
hal_m68k_interrupt_vsr in the file
hal_arch.S. When an interrupt occurs the original
68000 pushed the program counter and the status register on to the
stack, and then called the VSR via the exception table. On newer
variants some additional information is pushed, including details of
the interrupt source.
assumes the latter and can be used directly as the VSR on these newer
variants. On older variants a small trampoline is needed which pushes
the additional information and then jumps to the generic VSR.
Interpreting the additional information is handled via an assembler
which should be defined by the variant, matching the behaviour of the
hardware or the trampoline.
At the architecture level there is no fixed mapping between VSR and
ISR vectors. Instead that is left to the variant or platform HAL. The
architectural HAL does provide default implementations of
HAL_INTERRUPT_IN_USE since these just involve
updating a static table.
By default the interrupt state control macros
HAL_QUERY_INTERRUPTS are implemented by the
architectural HAL, and simply involve updating the status register.
Disabling interrupts involves setting the three IPL bits to 0x07.
Enabling interrupts involves setting those bits to a smaller value,
defaults to 0.
HAL_DISABLE_INTERRUPTS has no effect on
non-maskable interrupts. This causes a problem because parts of the
system assume that all normal interrupt sources are affected by this
macro. If the target hardware can raise non-maskable interrupts then
it is the responsibility of application code to install a suitable VSR
and handle non-maskable interrupts entirely within the application,
bypassing the usual eCos ISR and DSR mechanisms.
The architectural HAL does not provide any support for the interrupt
controller management macros like
HAL_INTERRUPT_MASK. These can only be implemented
on a per-variant, per-processor or per-platform basis.
Synchronous exception handling is done in much the same way as
interrupt handling. The architectural HAL provides a generic entry
hal_m68k_exception_vsr. On some variants
this can be used directly as the exception VSR, on others it will be
called via a small trampoline.
The details of exception handling vary widely from one variant to the next. Some variants push a great deal of additional information on to the stack for certain exceptions, but not all. The pushed program counter may correspond to the specific instruction that caused the exception, or the next instruction, or there may be only a loose correlation because of buffered writes. The architectural HAL makes no attempt to cope with all these differences, although some variants may provide more advanced support. Otherwise if an exception needs to be handled in a very specific way then it is up to the application to install a suitable VSR and handle the exception directly.
Stacks and Stack Sizes
values for minimal and recommended thread stack sizes,
CYGNUM_HAL_STACK_SIZE_TYPICAL. These values are
specific to the current configuration, and are affected mainly by
options related to interrupt handling.
By default eCos uses a separate interrupt stack, although this can be
disabled through the configuration option
When an interrupt or exception occurs eCos will save the context on
the current stack and then switch to the interrupt stack before
calling the appropriate ISR interrupt handler. This means that thread
stacks can be significantly smaller because there is no need to worry
about interrupt handling overheads, just the thread context. However
switching the stack does require some extra work and hence increases
the interrupt latency. Disabling the interrupt stack removes this
processing overhead but requires larger stack sizes. It depends on
the application whether or not this is a sensible trade off.
By default eCos does not allow nested interrupts, but this can be
controlled via the configuration option
Supporting nested interrupts requires larger thread stacks, especially
if the separate interrupt stack is also disabled.
Although the M68K has enough registers for typical operation, the calling conventions are memory-oriented. In particular all arguments are pushed on the stack rather than held in registers, and the return address is also pushed rather than ending up in a link register. To allow for this the recommended minimum stack sizes are a little bit larger than for some other architectures. Variant HALs cannot directly affect these stack sizes. However the sizes do depend in part on the size of a thread context, so if for example the processor supports hardware floating point and support for that is enabled then the stack sizes will increase.
Usually the M68K architectural HAL will provide a single block of
memory which acts as both the startup and interrupt stack, and there
are configuration options to
control the size of this block. Alternatively a variant, processor or
platform HAL may define either or both of
_HAL_M68K_INTERRUPT_STACK_BASE_ if for some reason
the stacks should not be placed in ordinary RAM.
Thread Contexts and Setjmp/Longjmp
A typical thread context consists of the following:
The integer context. This consists of the data registers
%d7and the address registers
%a6, The stack pointer register
%a7does not have to be saved explicitly since it is implicit in the pointer to the saved context.
The caller-save registers are
%a7and the status register. The remaining registers are callee-save. Function arguments are always passed on the stack. The result is held in
Floating point context, consisting of eight 64-bit floating point registers
%fp7and two support registers
%fpiar. Support for this is only relevant if the processor variant has a hardware floating point unit, and even then saving floating point context is optional and can be disabled using a configuration option
CYGIMP_HAL_M68K_FPU_SAVE. The control register
%fpcris not saved as part of the context. It is assumed that a single
0, will be used throughout the application.
The architectural HAL provides support for the hardware floating point unit. The variant or processor HAL should implement the CDL interface
CYGINT_HAL_M68K_VARIANT_FPUif this hardware unit is actually present.
Some M68K variants have additional hardware units, for example the
multiply-accumulate units in certain ColdFire processors. The
architectural HAL allows the context to be extended through various
macros such as
The status register
%srand the program counter. These are special because when an interrupt occurs the hardware automatically pushes these onto the stack, but exactly what gets pushed depends on the variant.
longjmp only deal
with the integer and fpu contexts. It is assumed that any special
hardware units will only be used by application code, not by the
compiler. Hence it is the responsibility of application code to
define and implement appropriate setjmp semantics for these units.
The variant HAL package can override the default implementations if
When porting to a new M68K variant, if this has a hardware floating
point unit then the variant HAL should implement the CDL interface
CYGINT_HAL_M68K_VARIANT_FPU, thus enabling support
provided by the architectural HAL. If the variant has additional
hardware units involving state that should be preserved during a
context switch or when an interrupt occurs, the variant HAL should
define a number of macros. The header file
cyg/hal/var_arch.h should define
HAL_CONTEXT_OTHER_INIT, either directly or via
assembler header file
cyg/hal/var.inc should define a number
of macros such as
For details of these macros see the architectural
Variants also need to define exactly how the status register and
program counter are saved onto the stack when an interrupt or
exception occurs. This is handled through C macros
HAL_CONTEXT_PCSR_INIT, and a number of assembler
macros such as
the architectural files
hal_arch.S provide more details of these.
For performance reasons the
HAL_MSBIT_INDEX macros are implemented using
assembler functions. A variant HAL can override the default
definitions if, for example, the variant has special instructions to
perform these operations.
Idle Thread Processing
is a no-op. A variant HAL may override this, for example to put the
processor into sleep mode. Alternative implementations should consider
exactly how this macro gets used in eCos kernel code.
The architectural HAL cannot provide the required clock support because it does not know what timer hardware may be available on the target hardware. Instead this is left to either the variant or platform HAL, depending on whether the processor has a suitable on-chip timer or whether an off-chip timer has to be used.
The M68K architecture does not have a separate I/O bus. Instead all hardware is assumed to be memory-mapped. Further it is assumed that all peripherals on the memory bus are wired appropriately for a big-endian processor and that there is no need for any byte swapping. Hence the various HAL macros for performing I/O simply involve pointers to volatile memory.
The variant, processor and platform equivalents of the
cyg/hal/hal_io.h header will typically
also provide details of some or all of the peripherals, for example
register offsets and the meaning of various bits in those registers.
If the processor has a cache then the variant HAL should implement the
causes the architectural header
cyg/hal/hal_cache.h to pick up
appropriate definitions from
cyg/hal/var_cache.h. The architectural
header will provide null defaults for anything not defined by the
The architectural HAL will generate the linker script for eCos
applications. This involves the architectural file
m68k.ld and a
layout file provided lower down, typically by the platform HAL. It is
the LDI file which specifies the types and amount of memory available
and which places code and data in appropriate places, but most of the
hard work is done via macros provided by the architectural
The architectural HAL does not implement diagnostic support. Instead this is left to the variant or platform HAL, depending on whether suitable peripherals are available on-chip or off-chip.
The M68K port does not have SMP support.
The M68K architectural HAL package provides basic support only for gdb stubs. There is no support for more advanced debug features like hardware watchpoints.
The generic gdb support in the common HAL requires a platform header
practice there is rarely any need for the contents of this file to
change between platforms so the architectural HAL can provide a
suitable default. It will do so if the CDL interface
The architectural HAL provides a default implementation of the
HAL_DELAY_US macro using a simply busy
loop. To use this support a lower-level HAL should define
_HAL_M68K_DELAY_US_LOOPS_, typically a small number
of about 20 but it will need to be calibrated during the porting
process. If the processor has a cache then the lower-level HAL may
the case when a delay loop is triggered while the cache is disabled.
The M68K architectural HAL implements the
function, allowing profiling tools like
gprof to determine the application's call
graph. It does not implement the profiling timer. Instead that
functionality needs to be provided by the variant or platform HAL.
The implementation of
mcount requires a dedicated
frame pointer register so code should be compiled without the
The M68K architectural HAL only implements the functionality provided by the eCos HAL specification and does not export any extra functionality.
|2021-10-22||Open Publication License|